Automotive bodies, household electric appliances and the like are produced by molding metallic materials, such as steel sheets and galvanized steel sheets, into metallic moldings, followed by coating and assembling, etc. The coating of such metallic moldings includes various steps, namely the steps of degreasing, surface conditioning, chemical conversion treatment, electrodeposition coating, among others.
The surface-conditioning treatment is a treatment required for enabling, in the next phosphate coating treatment step, rapid formation of uniform and high-density coatings consisting of phosphate salt crystals all over the metal surface. It generally comprises immersing metallic moldings in a surface-conditioning bath to thereby cause formation of phosphate crystal nuclei on the metal surface.
The surface-conditioning ability of a surface-conditioning agent results from the titanium phosphate colloid contained in the surface-conditioning agent. The titanium phosphate colloid by nature tends to undergo dissolution or aggregation with the lapse of time, hence be progressively deteriorated.
In the case of surface-conditioning agents, techniques have been investigated for stabilizing the titanium phosphate colloid by pII adjustment and other means to thereby avoid the degradation of colloidal character or the unsatisfactory surface conditioning resulting from the colloid becoming a suspension.
Japanese Kokai Publication Hei-02-83202 discloses a method for preparing activated titanium phosphate having stable surface conditioning ability and suited for zinc phosphating. Said method comprises preparing activated titanium phosphate for zinc phosphating from a titanium (IV) compound and at least one member selected from the group consisting of an orthophosphoric acid compound, a polyphosphoric acid compound and a metaphosphoric acid compound and can give a surface-conditioning agent having stable surface-conditioning ability as compared with the prior art surface-conditioning agents showing greatly varying surface-conditioning ability among batches even when prepared under certain specified reaction conditions.
The surface-conditioning agent obtained by said method, however, still has drawbacks. For instance, the pH control in surface-conditioning baths is difficult, hence it is difficult to obtain a stable titanium phosphate colloid, and this is disadvantageous in particular in dipping treatment which requires a stable surface-conditioning agent in large quantities over a long period of time.
Japanese Kokoku Publication Sho-58-55229 discloses a surface-conditioning agent for zinc phosphating which comprises titanium ion, phosphate ion, pyrophosphate ion and carbonate ion. This surface-conditioning agent is characterized in that the pH is prevented from decreasing by the addition of a carbonate ion-providing carbonic acid compound and the titanium phosphate colloid is stabilized by the addition of pyrophosphate ion and that the surface-conditioning ability can be stably exhibited over a long period of time. Another feature is that said composition is advantageous in dipping treatment in which a, particular requirement is long-period stability, since the pH can easily be controlled by addition of a carbonic acid compound.
However, the pyrophosphate ion used there is not sufficient in its property as a metal chelating agent capable of stabilizing titanium phosphate colloid and said surface-conditioning agent cannot be said to be a surface-conditioning agent having good durability.